1. Field
The present invention relates generally to communications, and more specifically, to systems and techniques for managing code channel assignments in a wireless communications system.
2. Background
Modern communications systems are designed to allow multiple users to share a common communications medium. One such communications system is a Code Division Multiple Access (CDMA) system. The CDMA communications system is a modulation and multiple access scheme based on spread-spectrum communications. In a CDMA communications system, a large number of signals share the same frequency spectrum and, as a result, provide an increase in user capacity. This is achieved by transmitting each signal with a different code that modulates a carrier, and thereby, spreads the signal over the entire spectrum. The transmitted signals may be separated in the receiver by a correlator using a corresponding code to de-spread the desired signal. The undesired signals, whose codes do not match, contribute only to noise.
In spread-spectrum communications, fixed base stations are generally dispersed throughout an access network to support wireless communications with various user devices. The access network may be divided into regions known as cells with a base station serving each cell. In high traffic applications, the cell may be further divided into sectors with a base station serving each sector. In this configuration, the base station may assign one or more dedicated channels using Walsh codes to each user within its cellular region to support voice and data communications over a forward link transmission. A forward link transmission refers to a transmission from the base station to a user and a reverse link transmission refers to a transmission from a user to the base station. One or more shared channels may also be used by the base station with its own distinct Walsh code. Additional Walsh code assignments may be reserved for various signaling and system support functions.
There is a limited number of Walsh codes available to any given base station, and thus the number of channels, including dedicated and shared channels, is limited for a given code space. In CDMA systems of the past, the forward link capacity was limited by the mutual interference between multiple users, and therefore, the code space was sufficient for the number of supportable channels. However, recent advances in technology has reduced the effects of interference, allowing for additional simultaneous users, and thus increasing demand for more codes to support additional channels.
Moreover, with the tremendous increase in wireless communications over the past years, there has been an ever increasing demand for higher data rate services to support web browsing, video applications, and the like. Often this demand is met by using multiple dedicated channels to carry data from the base station to the user with each channel having a distinct Walsh code. In some instances, high data rate services may be supported by variable Walsh spreading. Variable Walsh spreading entails using shorter length Walsh codes for higher data rate transmissions. Using a shorter length Walsh code, however, precludes using all longer codes that contain the chip pattern of the shorter code, thereby depleting multiple Walsh codes.
Increased demand for codes, reduced available codes, or a combination of the two could result in an insufficient number of Walsh codes to channelize the forward link. Thus, the capacity of the system may be limited in situations where, due to advances in interference mitigation, additional users and/or increased data throughput might otherwise be available. Accordingly, there is a need in the art for an efficient methodology for managing code allocations.